[Richard Crowley]

Zoom H4N gives you two choices:
44.1 K = audio recording for audio production
48K = audio recording for video production.
Makes no difference what the video frame rate is.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

It would be much easier and faster to do this with the schematic diagram of the car stereo you are trying to hack. We don’t really know where in the circuit you are injecting your aux input. According to the data sheet you cited, the LC75421M device actually has 10 inputs (5 Left and 5 Right channel).

If your signal level is too low, then maybe you removed too much of the tape preamp circuit. There could be several causes of distortion, but impossible to diagnose without better details of the circuit. It is quite likely that the different signal levels from different sources (radio, tape, etc.) are “normalized” in firmware in the car stereo. It is programmed in that the tape preamp is perhaps x.x dB higher than the radio, so when you switch inputs, it automatically adjusts the gain. This makes it easy for them to do in software, but nearly impossible to hack after the fact.

As I said before, there are actually five stereo inputs on that chip. But the ability to select any more than the ones already implemented are locked away in the firmware of the car stereo. You can see the pin numbers of the inputs on page 3 of the data sheet.

I don’t see anything like your circuit fragment in the data sheet? C2 is a crude low-pass filter to keep noise out of the audio. R1/R2 form a fixed L-pad attenuator to reduce the signal level, and C1 is a simple DC-blocking (AC-coupling) capacitor to keep external DC from interfering with the chip’s internal circuits, and likely to allow the chip internally to use a “virtual reference ground” of (“Vref”) of VDD/2 (around 6.2V)

You will see in the data sheet that Sanyo recommends 1uF for “C1” as the DC-blocking input capacitor.

Note that this is really an electronics question and not really very appropriate for this forum. The best forum on the internet for electronics is https://www.eevblog.com/forum/ I don’t think there’s much traffic anymore on Usenet (rec.audio.pro, et.al.) for the last few decades.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

The Sony ECM-674 is rather a “lower-end” product they seem to offer mostly as an on-camera mic. There probably aren’t as many “huge strides in shotgun mic builds” as you would think. Most of the effort seems to have been in lower-end products, i.e. 3.5mm, non P48 phantom power models. (Like all those little plastic Rode things, etc.)

As Ty Ford suggests, you could “up-scale” with something higher-end. But, if you got satisfactory performance from your ECM-674, perhaps just replacing it with the same model seems like a logical solution. There are currently a B-stock ECM-674s on Ebay for $180. That’s what I would do if it were me.

Ref: https://www.ebay.com/itm/161756119639

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Unless you have an unusually portable and field-friendly combination of computer, keyboard, mouse, screen and audio interface, virtually all people who have actually tried it find that using a computer and audio interface in a video/cine location set has far too few (if any?) advantages to outweigh the considerable disadvantages. Field audio recording devices are much more robust and field-friendly than ANY computer and audio interface combination.

Exceptions are where you are working on a sound cart where you have all your gear set up (interfaces, mixer, wireless receivers, cable and boom storage, big battery power supplies, etc. etc. There are SOME sound recording applications that are specifically made for production field recording. The prime example being “Boom Recorder”. This is a “minimalist subset DAW” specifically customized for field production TRACKING. (Please note well the difference between TRACKING and EDITING). Boom Recorder is optimized for multi-track recording in situations like field production. Your typical DAW application becomes rather fiddly in hectic field production situations.

It is not at all clear how using a DAW will help you “isolate” or “spot” unwanted noise. And out in the field (and in the presence of the same noise) is possibly the WORST possible place to try to remove it. And recording ADR out in the field is also a very questionable practice because frequently the reason you need to record ADR is because of the ambient noise, so trying to record ADR in the SAME ambient noise just seems like a losing proposition.

Ref: https://www.vosgames.nl/products/BoomRecorder/

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Phantom power insertion isn’t rocket science. IMHO, the $20-30 units are sensibly priced for what they are. The gadgets they are selling for $50 and up are overpriced (IMHO). They have to pay for those catalogs. And the “brand name” that is screen-printed on the generic box out of the same factory, which is why they all look the same.

When it comes down to it. It is just a box with a tiny handful of passive components and some connectors. If you are worried about it, buy from a vendor with good return policies. Or pay double for the privilege of going down to your neighborhood guitar shop, etc.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Have you confirmed that you are picking up the sound from the microphone and not from the internal microphone in your device? That could be as simple as tapping on the microphone with your fingernail to isolate the mechanical sound and eliminate confusion of sound going through the air.

We assume that you have properly installed the included coin-cell battery in the “line-lump”. Does it work properly if you plug it into some other device?

What happens if you move the mic around when you are talking. If you aim it right at your mouth from 6~8 inches away and hold it there does it sound better? Are you putting it so close to your neck that you are experiencing “chin shading”

For the same purpose, I bought this “IK Multimedia iRig Mic Lav compact lavalier microphone for smartphones and tablets”. A major selling factor for me was that it allows you to plug in your headphones (or earbuds) so that you can HEAR what the microphone is doing. That meets the requirements of my signature phrase below….

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

The electret mic capsule requires that DC voltage to operate. Your circuit has capacitor C1 which isolates the microphone from the DC voltage it requires.

You could start by removing C1 and connect the mic capsule directly to the base and R1. However, since there are so many unknowns here, the value of 470K ohms for R1 may very possibly be too high. It may require a much lower resistance, perhaps more like 47K or even lower to match the impedance of the mic capsule while properly biasing Q1 for linear operation.

You may need a more sophisticated version of this kind of circuit…

But that circuit assumes a separate power supply. It might be possible to modify the circuit to use the “plug-in power” from the camera, but it may be beyond the scope of your “dirty prototype” limits.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

With the exception of that Zoom SGH-6 micropnhone, NONE of the little hand-held recorders have particularly directional microphones. Most are nominally “cardioid”, but probably not even as directional as a Shure SM57. Some might work OK if you used the recorder itself in “hand-held” mode and held the built-in mic up to the subject (no more than 12 inches, or even closer in noisy situations). But that becomes a bit unwieldy holding the whole recorder up.

I have heard others say that they didn’t want to use hand-held mics “for esthetic reasons”. but I have never heard any justification for that that made any sense to me. If you want to get good SNR in a noisy ambient, then you must use a very directional mic or else get the mic pretty close to the subject. There is no shortcut around the laws of acoustic physics. Proximity effect is your friend.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

Its not the recorder that can improve your “signal-to-noise ratio” (SNR). It is the microphone(s).

You did not reveal whether this is “run-n-gun”, hand-held brief encounters, or whether this is more formal sit-down interview style. You also did not reveal whether this is a dialog between the interviewer and the subject, or whether you only need to capture the subject’s dialog?

Getting the microphone close to the talker’s mouth is an excellent way of increasing the SNR. Using headset/earset style microphones will get the mic as close to the talker’s mouth as possible. And that may be the best way of picking up the interviewer. But it is not necessarily the easiest way to mic a subject.

Using a very directional (hyper-cardioid or “shotgun”) microphone is another way of increasing the SNR. There is even a small, portable recorder with a shotgun mic attachment. See the Zoom H-6 with the SGH-6 microphone.

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.

[Richard Crowley]

If the feedback frequency(s) is STABLE (doesn’t change in frequency) then you could use a “notch filter” to remove that feedback frequency from being amplified. This is the method used in large sound reinforcement (“PA”) systems. There are sophisticated filters which automatically detect feedback frequencies and attenuate them from being amplified.

You did not reveal what this project is supposed to do, but using a microphone and speaker in close proximity practically guarantees acoustic feedback problems. Might there be some other way of preventing the feedback?

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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.